The present invention relates to improvements over a magnet ignition device of the electronic type.
A set of timing charts, shown in FIG. 1, illustrating the operation principle of a known ignition device disclosed in Japan published unexamined patent application No. 36234/77 (52 of Showa) well illustrates particularly a case where the rotational frequency of an engine or an angular velocity .omega. of the crankshaft is remarkably reduced.
In FIG. 1, M1 and M2 on a time axis (a) represent two different rotational angular positions of the crankshaft; T a top dead point; S a required ignition angular position. Triangle waveforms lying on a time axis (b) show variations of charging and discharging voltages of the capacitor used in the device of the above-mentioned specification. Vref designates a reference voltage for determining the charging start and the discharging termination. The device is so arranged that the capacitor is charged during a period from the position M1 to the position M2 with a current i.sub.1 and following the position M2 it is discharged with a current i.sub.2, and at the reference voltage Vref an ignition signal is issued. A curve decepted above and along a time axis (c) shown in FIG. 1 generally illustrates a variation of the rotational angular velocity .omega. of the engine.
Let us assume that an angle and a lapse of time between the positions M1 and M2 are .theta.1 and T1, an angle and a time lapse between positions M2 and S are .theta.2 and T2, an angle between positions S and T is .alpha., and an angle between the position T and the next position M1 is .theta.3. On the assumption, an advance angle .alpha. is given by EQU .alpha.=180-(.theta.1+.theta.2+.theta.3) (1)
In the equation, .theta.1 and .theta.3 are constant, which are in dependence upon the positions M1 and M2 on the rotational angle axis of the crankshaft, and .theta.2 is expressed EQU .theta.=.omega.2 T2 (2)
where .omega.2 is an average angular velocity over time point t2 to t3. Accordingly, the advance angle .alpha. may be expressed EQU .alpha.=K-.omega.2 T2 (3)
where K is a constant and is given EQU K=180-.theta.1-.theta.3 (4)
The amounts of the charging and discharging of the capacitor are fixed, so that we have EQU i.sub.1 T1=i.sub.2 T2 (5)
T1 may also be expressed EQU T1=.theta.1/.omega.1 (6)
From the equations (3), (5) and (6), we have EQU .alpha.=K-i.sub.1 /i.sub.2 .times..omega.2/.omega.1.times..theta.1 (7)
As seen from the equation (7), if i.sub.1 or i.sub.2 is changed in accordance with a running condition of the engine, the advance angle changes corresponding to the change of the current.
As described above, if the advance angle .alpha. is adjusted on the basis of the equation (7), the ratio .omega.2/.omega.1 is always constant. Let us consider a case where, when a spark is produced at time point t7, for example, it fails to ignite a combustion mixture in the engine cylinder. In this case, the rotational speed of the crankshaft rapidly decreases and a time till the top dead point T and the next angular position M1 are considerably elongated, and further times T1' and T2' substantially corresponding to the ignition preparing period of another stroke following the present one is considerably elongated. As a result, if a ratio of an average angular velocity .omega.'2 from time point t10 to t11 to an average angular velocity .omega.'1 from time point t9 to t10, i.e. .omega.'2/.omega.'1, is smaller than the ratio .omega.2/.omega.1 in the preceding stroke, the equation (7) shows that, even if the current ratio i.sub.1 /i.sub.2 is constant, .alpha. grows. As a result, the spark is produced at an angular position S' far advanced relative to the position S for the required ignition time.
As the rotational speed of the engine is lower, a mixing condition of the intake combustion mixture is generally worse, so that the irregular combustion or the ignition failure is apt to occur, and thus a variation of the rotational frequency is great.
To fix the advanced angle .alpha. in low rotational frequencies of the engine crankshaft, even if the charging and discharging currents of the capacitor can be kept constant, when the angular velocity .omega. greatly changes every cycle as shown in FIG. 1, the ratio .omega.2/.omega.1 changes every moment, so that the actual ignition timing varies relative to the required ignition time S. A stable and accurate ignition timing is never obtained.